Nonvolatile ferroelectric memory device

ABSTRACT

A nonvolatile ferroelectric memory device is provided so as to control read/write operations of a nonvolatile memory cell using a channel resistance of the memory cell which is differentiated by polarity states of a ferroelectric material. In the memory device, an insulating layer is formed on a bottom word line, and a floating channel layer comprising a N-type drain region, a P-type channel region and a N-type source region is formed on the insulating layer. Then, a ferroelectric layer is formed on the floating channel layer, and a word line is formed on the ferroelectric layer. As a result, the resistance state induced to the channel region is controlled depending on the polarity of the ferroelectric layer, thereby regulating the read/write operations of the memory cell array.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a nonvolatile ferroelectricmemory device, and more specifically, to a technology of controllingread/write operations of a nonvolatile ferroelectric memory cell using achannel resistance of a memory cell which is differentiated by polaritystates of a ferroelectric material in a nano scale memory device.

2. Description of the Related Art

Generally, a ferroelectric random access memory (hereinafter, referredto as ‘FeRAM’) has attracted considerable attention as next generationmemory device because it has a data processing speed as fast as aDynamic Random Access Memory (hereinafter, referred to as ‘DRAM’) andconserves data even after the power is turned off.

The FeRAM having structures similar to the DRAM includes the capacitorsmade of a ferroelectric substance, so that it utilizes thecharacteristic of a high residual polarization of the ferroelectricsubstance in which data is not deleted even after an electric field iseliminated.

The technical contents on the above FeRAM are disclosed in the KoreanPatent Application No. 2001-57275 by the same inventor of the presentinvention. Therefore, the basic structure and the operation on the FeRAMare not described herein.

A unit cell of a conventional nonvolatile FeRAM device comprises aswitching element and a nonvolatile ferroelectric capacitor. Theswitching element performs a switching operation depending on a state ofa word line to connect a nonvolatile ferroelectric capacitor to a subbit line. The nonvolatile ferroelectric capacitor is connected betweenand a plate line and one terminal of the switching element.

Here, the switching element of the conventional FeRAM is a NMOStransistor whose switching operation is controlled by a gate controlsignal.

FIG. 1 is a cross-sectional diagram illustrating a conventionalnonvolatile ferroelectric memory device.

A conventional 1-T (One-Transistor) FET (Field Effect Transistor) cellcomprises a N-type drain region 2 and a N-type source region 3 which areformed on a P-type region substrate 1. A ferroelectric layer 4 is formedon a channel region, and a word line 5 is formed on the ferroelectriclayer 4.

The above-described conventional nonvolatile FeRAM device reads andwrites data by using a channel resistance of the memory cell which isdifferentiated depending on polarization states of the ferroelectriclayer 4. That is, when the polarity of the ferroelectric layer 4 inducespositive charges to the channel, the memory cell becomes at a highresistance state to be turned off. On the contrary, when the polarity ofthe ferroelectric layer 4 induces negative charges to the channel, thememory cell becomes at a low resistance state to be turned on.

However, in the conventional nonvolatile FeRAM device, when the cellsize becomes smaller, a data maintaining characteristic is degraded, sothat it is difficult to perform the normal operation of the cell. Thatis, a voltage is applied to an adjacent cell at read/write modes todestroy data of unselected cells, so that interface noise is generatedbetween the cells and it is difficult to perform a random accessoperation.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to form a floatingchannel layer comprising a N-type drain region, a P-type channel regionand N-type source region between a word line and a bottom word line,thereby improving a data maintaining characteristic.

It is another object of the present invention to provide theabove-described memory cell so as to control read/write operations of amemory cell array, thereby improving reliability of the cell andreducing the whole size of the cell at the same time.

In an embodiment, a nonvolatile ferroelectric memory device comprises aninsulating layer formed on a bottom word line, a floating channel layer,a ferroelectric layer formed on the channel region of the floatingchannel layer, and a word line formed on the ferroelectric layer. Thefloating channel layer comprises a P-type channel region formed on theinsulating layer and kept at a floating state, a N-type drain region anda N-type source region which are connected to both sides of the channelregion. Here, different channel resistance is induced to the channelregion depending on polarity states of the ferroelectric layer, so thatdata are read and written.

In another embodiment, a nonvolatile ferroelectric memory devicecomprising a unit cell array which includes a plurality of bottom wordlines, a plurality of insulating layers, a floating channel layer, aplurality of ferroelectric layers, a plurality of word lines. Theplurality of insulating layers are formed on the plurality of bottomword lines, respectively. The floating channel layer comprises aplurality of P-type channel regions located on the plurality ofinsulating layers and a plurality of N-type drain and source regionswhich are alternately connected in series to the plurality of P-typeregions. The plurality of ferroelectric layers are formed respectivelyon the plurality of P-type channel regions of the floating channellayer. The plurality of word lines are formed on the plurality offerroelectric layers, respectively. Here, the unit cell array reads andwrites a plurality of data by inducing different channel resistance tothe plurality of P-type channel regions depending on polarity states ofthe plurality of ferroelectric layers.

In still another embodiment, a nonvolatile ferroelectric memory devicecomprising a unit cell array which includes a bottom word line, aplurality of insulating layers, a floating channel layer, a plurality offerroelectric layers, and a plurality of word lines. The plurality ofinsulating layers are formed on the bottom word line, respectively. Thefloating channel layer comprises a plurality of P-type channel regionslocated on the plurality of insulating layers and a plurality of N-typedrain and source regions which are alternately connected in series tothe plurality of P-type channel regions. The plurality of ferroelectriclayers are formed respectively on the plurality of P-type channelregions of the floating channel layer. The plurality of word lines areformed on the plurality of ferroelectric layers, respectively. Here, theunit cell array reads and writes a plurality of data by inducingdifferent channel resistance to the plurality of P-type channel regionsdepending on polarity states of the plurality of ferroelectric layers.

In still another embodiment, a nonvolatile ferroelectric memory devicecomprises a bottom word line, a floating channel layer formed on thebottom word line and kept at a floating state, a ferroelectric layer,and a word line formed on the ferroelectric layer in parallel with thebottom word line. The ferroelectric layer where data are stored isformed on the floating channel layer. Here a write operation isperformed on data corresponding to the ferroelectric layer depending ona state of a voltage level applied to the bottom word line and the wordline, and a read operation is performed on data by inducing differentchannel resistance to the floating channel layer depending on polaritystates of charges stored in the ferroelectric layer.

In still another embodiment, a nonvolatile ferroelectric memory devicecomprises a plurality of memory cells, a first switching element, and asecond switching element. Switching operations of the plurality ofmemory cells are selectively controlled respectively depending onvoltages applied to a plurality of word lines and a plurality of bottomword lines, floating channel layers are connected serially in theplurality of memory cells. The first switching element selectivelyconnects the plurality of memory cells to a bit line in response to afirst selecting signal. The second switching element selectivelyconnects the plurality of memory cells to a sensing line in response toa second selecting signal. Here, each of the plurality of memory cellscomprises an insulating layer formed on the bottom word line, thefloating channel layer, a ferroelectric layer formed on the channelregion of the floating channel layer, and a word line formed on theferroelectric layer. The floating channel layer comprises a P-typechannel region formed on the insulating layer and kept at a floatingstate, a N-type drain region and a N-type source region which areconnected to both sides of the channel region.

In still another embodiment, a nonvolatile ferroelectric memory devicecomprises a plurality of bit lines arranged in a row direction, aplurality of sensing lines arranged perpendicular to the plurality ofbit lines, a plurality of memory cells arranged in row and columndirections where the plurality of bit lines and the plurality of sensinglines are crossed, and a plurality of sense amplifiers connected one byone to the plurality of bit lines. Here, each of the plurality of memorycells comprises an insulating layer formed on a bottom word line, thefloating channel layer comprising a P-type channel region formed on thefloating layer and kept at a floating state, a N-type drain region and aN-type source region which are connected to both sides of the channelregion, a ferroelectric layer formed on the channel region of thefloating channel layer, and a word line formed on the ferroelectriclayer.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects and advantages of the present invention will becomeapparent upon reading the following detailed description and uponreference to the drawings in which:

FIG. 1 is a cross-sectional diagram illustrating a conventionalnonvolatile ferroelectric memory device;

FIGS. 2 a to 2 c are diagrams illustrating a cross section of a cell andits symbol of a nonvolatile ferroelectric memory device according to anembodiment of the present invention;

FIGS. 3 a and 3 b are diagrams illustrating write and read operations onhigh data of the nonvolatile ferroelectric memory device according to anembodiment of the present invention;

FIGS. 4 a and 4 b are diagrams illustrating write and read operation onlow data of the nonvolatile ferroelectric memory device according to anembodiment of the present invention;

FIG. 5 is a layout cross-sectional diagram illustrating the nonvolatileferroelectric memory device according to an embodiment of the presentinvention;

FIGS. 6 a and 6 b are cross sectional diagrams illustrating thenonvolatile ferroelectric memory device according to an embodiment ofthe present invention;

FIG. 7 is a cross-sectional diagram illustrating a nonvolatileferroelectric memory device having a multiple layer structure accordingto an embodiment of the present invention;

FIG. 8 is a diagram illustrating a nonvolatile ferroelectric memorydevice according to an embodiment of the present invention;

FIGS. 9 a and 9 b are cross-sectional diagrams illustrating thenonvolatile ferroelectric memory device of FIG. 8;

FIG. 10 is a cross-sectional diagram illustrating a nonvolatileferroelectric memory device having a multiple layer structure of FIG. 8;

FIG. 11 is a diagram illustrating a unit array of the nonvolatileferroelectric memory device according to an embodiment of the presentinvention;

FIG. 12 is a diagram illustrating a nonvolatile ferroelectric memorydevice according to an embodiment of the present invention;

FIG. 13 is a diagram illustrating a write operation of the nonvolatileferroelectric memory according to an embodiment of the presentinvention;

FIG. 14 is a timing diagram illustrating a write operation of high datain the nonvolatile ferroelectric memory device according to anembodiment of the present invention;

FIG. 15 is a timing diagram illustrating a write operation of low dataand high data maintenance in the nonvolatile ferroelectric memory deviceaccording to an embodiment of the present invention; and

FIG. 16 is a timing diagram illustrating a sensing operation of celldata in the nonvolatile ferroelectric memory device according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail with reference to theaccompanying drawings.

FIGS. 2 a to 2 c are diagrams illustrating a cross section of a cell andits symbol of a nonvolatile ferroelectric memory device according to anembodiment of the present invention.

FIG. 2 a is a cross-sectional diagram illustrating a unit cell in adirection in parallel with a word line.

A bottom word line 10 formed in the bottom layer of the unit cell isarranged in parallel with a word line 17 formed in the top layer of theunit cell. Here, the bottom word line 10 and the word line 17 areselectively driven by the same row address decoder (not shown). An oxidelayer 11 is formed on the bottom word line 10, and a floating channellayer 15 comprising a P-type channel region 12 is formed on the oxidelayer 11.

A ferroelectric layer 16 is formed on the floating channel layer 15, andthe word line 17 is formed on the ferroelectric layer 16.

FIG. 2 b is a cross-sectional diagram illustrating a unit cell in adirection perpendicular to the word line.

The oxide layer 11 is formed on the bottom word line 10. The floatingchannel layer 15 is formed on the insulating layer 11. Here, a drainregion 13 and a source region 14 are formed with a N-type, and thechannel region 12 is formed with a P-type in the floating channel layer15, which becomes at a floating state.

For a semiconductor of the floating channel layer 15, materials such asa carbon nano tube, silicon and Ge (Germanium) can be used. Theferroelectric layer 16 is formed on the P-type channel region 12 of thefloating channel layer 15, and the word line 17 is formed on theferroelectric layer 16.

As a result, the nonvolatile ferroelectric memory device according to anembodiment of the present invention reads and writes data by using achannel resistance of the floating channel layer 15 which isdifferentiated by polarization states of the ferroelectric layer 16.That is, when the polarity of the ferroelectric layer 16 inducespositive charges to the channel region 12, the memory cell becomes at ahigh resistance state, so that the channel is turned off. On thecontrary, when the polarity of the ferroelectric layer 16 inducesnegative charges to the channel region 12, the memory cell becomes at alow resistance state, so that the channel is turned on.

The above-described unit memory cell according to the embodiment of thepresent invention is represented by a symbol shown in FIG. 2 c.

FIGS. 3 a and 3 b are diagrams illustrating write and read operations onhigh data of the nonvolatile ferroelectric memory device according to anembodiment of the present invention.

Referring to FIG. 3 a, when data “1” is written, a positive voltage <+V>is applied to the bottom word line 10, and a negative voltage <−V> isapplied to the word line 17. Here, the drain region 13 and the sourceregion 14 are become at a ground voltage <GND> state.

In this case, a voltage is applied between the ferroelectric layer 16and the P-type channel region 12 of the floating channel layer 15 byvoltage division of a capacitor between the ferroelectric layer 16 andthe oxide layer 11.

Then, positive charges are induced to the channel region 12 depending onthe polarity of the ferroelectric layer 16, so that the memory cellbecomes at the high resistance state. Here, since the positive chargesare induced to the channel region 12, and the drain region 13 and thesource region 14 are at the ground state, the channel region 12 is keptoff. As a result, the data “1” is written in all memory cells at thewrite mode.

Referring to FIG. 3 b, when the data “1” is read, the ground voltage<GND> is applied to the bottom word line 10 and the word line 17. Here,since the positive charges are induced to the channel region 12, and thedrain region 13 and the source region 14 are at the ground state, thechannel region 12 is kept off.

As a result, at the read mode, the data “1” stored in the memory cell isread. Here, when a low voltage difference is applied to the drain region13 and the source region 14, small current flows because the channelregion 12 is turned off.

FIGS. 4 a and 4 b are diagrams illustrating write and read operation onlow data of the nonvolatile ferroelectric memory device according to anembodiment of the present invention.

Referring to FIG. 4 a, when data “0” is written, a positive voltage <+V>is applied to the bottom word line 10 and the word line 17. Here, thedrain region 13 and the source region 14 are become at a ground voltage<GND> state.

In this case, since negative charges are induced to the channel region12, and the drain region 13 and the source region 14 are at the groundstate, the channel region 12 is kept on. As a result, the channel region12 is turned on, so that a ground voltage flows.

A high voltage is formed between the ground voltage formed in thechannel region 12 and the positive voltage <+V> applied from the wordline 17. Then, negative charges are induced to the channel region 12depending on the polarity of the ferroelectric layer 16, so that thememory cell becomes at a low resistance state. As a result, the data “0”is written in the memory cell at the write mode.

Meanwhile, while the data “1” is stored, the positive voltage <+V> isapplied to the drain region 13 and the source region 14. When thepositive voltage <+V> is applied to the bottom word line 10 and the wordline 17, the channel region 12 is turned off. As a result, the groundvoltage cannot flow in the channel region 12.

In this case, a voltage difference is not generated between the positivevoltage of the channel region 12 at the floating state and the positivevoltage <+V> of the word line 17. Thus, the polarity change of theferroelectric layer 16 is not generated but the previous polarity statecan be maintained. As a result, the data “0” is written in the cellselected after the data “1” is written.

Referring to FIG. 4 b, when the data “0” is read, the ground voltage<GND> is applied to the bottom word line 10 and the word line 17. Here,since the channel region 12 is turned on even when a low voltagedifference is applied between the drain region 13 and the source region14, a large amount of current can flow. As a result, the data “0” storedin the memory cell is read at the read mode.

Therefore, at the read mode, the data maintaining characteristic of thecell can be improved because the word line 17 and the bottom word line10 are controlled at the ground level so that a voltage stress is notapplied to the ferroelectric layer 16.

FIG. 5 is a layout cross-sectional diagram illustrating the nonvolatileferroelectric memory device according to an embodiment of the presentinvention.

In the embodiment, a plurality of word lines WL are arranged in parallelwith a plurality of bottom word lines BWL in a column direction. Aplurality of bit lines BL are arranged perpendicular to the plurality ofword lines WL. Also, a plurality of unit cells C are located where theplurality of word lines WL, the plurality of bottom word lines BWL andthe plurality of bit lines BL are crossed.

FIGS. 6 a and 6 b are cross sectional diagrams illustrating thenonvolatile ferroelectric memory device according to an embodiment ofthe present invention.

FIG. 6 a is a diagram illustrating a cross section of the cell array ina direction (A) in parallel with the word line WL of FIG. 5.

In the cell array according to the embodiment of the present invention,a plurality of oxide layers 11 are formed on the bottom word line 10,and a plurality of P-type channel regions 12 are formed on the pluralityof oxide layers 11. A plurality of ferroelectric layers 16 are formed onthe plurality of channel regions 12, and the word line 17 is formed inparallel with the bottom word line 10 on the plurality of ferroelectriclayers 16. As a result, a plurality of cells are connected between oneword line WL_1 and one bottom word line BWL_1.

FIG. 6 b is a diagram illustrating a cross section of the cell array ina direction (B) perpendicular to the word line WL of FIG. 5.

In the cell array according to the embodiment of the present invention,the oxide layer 11 is formed on each bottom word line BWL_1, BWL_2 andBWL_3. The floating channel layer 15 comprising the N-type drain region13, the P-type channel region 12 and the N-type source region 14connected serially is formed on the oxide layer 11.

Here, the N-type drain region 13 can be used as a source region in theadjacent cell, and the N-type source region 14 can be used as a drainregion in the adjacent cell. That is, the N-type region is used incommon as a drain region and a source region in the adjacent cell.

The ferroelectric layer 16 is formed on each channel region 12 of thefloating channel layer 15, and the word lines WL_1, WL_2 and WL_3 areformed on the ferroelectric layer 16.

FIG. 7 is a cross-sectional diagram illustrating a nonvolatileferroelectric memory device having a multiple layer structure accordingto an embodiment of the present invention.

Referring to FIG. 7, the unit cell array shown in FIG. 6 b is depositedas a multiple layer structure. Each unit cell array is separated by theoxide layer 18.

FIG. 8 is a diagram illustrating a nonvolatile ferroelectric memorydevice according to another embodiment of the present invention.

In another embodiment, the bottom word line 10 is used in common in apredetermined cell array. A plurality of word lines WL are arranged in acolumn direction, and a plurality of bit lines BL are arranged in a rowdirection. A plurality of unit cells C are located where the pluralityof word lines WL and the plurality of BL are crossed.

FIGS. 9 a and 9 b are cross-sectional diagrams illustrating thenonvolatile ferroelectric memory device of FIG. 8.

FIG. 9 a is a diagram illustrating a cross section of a cell array in adirection (C) in parallel with the word line WL of FIG. 8.

In the cell array according to the embodiment of the present invention,a plurality of oxide layers 11 are formed on the bottom word line 10,and a plurality of P-type channel regions 12 are formed on the pluralityof insulating layers 11. A plurality of ferroelectric layers 16 areformed on the plurality of channel regions 12, and the word line 17 isformed in parallel with the bottom word line 10 on the plurality offerroelectric layers 16. Thus, a plurality of cells are connectedbetween one word line WL_1 and one bottom word line BWL_1.

FIG. 9 b is a diagram illustrating a cross section of a cell array in adirection (D) perpendicular to the word line WL of FIG. 8.

In the cell array according to the embodiment of the present invention,the oxide layer 11 is formed on the bottom word lines BWL_1, BWL_2 andBWL_3 connected in common. The floating channel layer 15 comprising theN-type drain region 13, the P-type channel region 12 and the N-typesource region 14 is formed on the oxide layer 11. The ferroelectriclayer 16 is formed on each channel region 12 of the floating channellayer 15, and the word lines WL_1, WL_2 and WL_3 are formed on theferroelectric layer 16.

FIG. 10 is a cross-sectional diagram illustrating a nonvolatileferroelectric memory device having a multiple layer structure of FIG. 8.

Referring to FIG. 10, the unit cell array shown in FIG. 9 b is depositedas a multiple layer structure. Each unit cell array is separated by theoxide layer 18.

FIG. 11 is a diagram illustrating a unit array of the nonvolatileferroelectric memory device according to an embodiment of the presentinvention.

In the embodiment, the unit array of FIG. 11 comprises switchingelements N1, N2 and a plurality of memory cells Q1˜Qm. Here, theswitching element N1, which is connected between the bit line BL and thememory cell Q1, has a gate to receive a selecting signal SEL_1. Theswitching element N2, which is connected between a sensing line S/L andthe memory cell Qm, has a gate to receive a selecting signal SEL_2.

The plurality of memory cells Q1˜Qm, which are connected seriallybetween the switching elements N1 and N2, selectively perform aswitching operation by word lines WL_1˜WL_m and bottom word linesBWL_1˜BWL_m. The detailed structure of each memory cell Q1˜Qm is shownin FIG. 2 b. Thus, a source of the memory cell Q1 is connected to theswitching element N1, and a drain of the memory cell Qm is connected tothe switching element N2.

FIG. 12 is a diagram illustrating a nonvolatile ferroelectric memorydevice according to an embodiment of the present invention.

In the embodiment, a plurality of unit cell arrays 20˜23 are arrangedwhere a plurality of bit lines BL_1˜BL_n and a plurality of sensinglines S/L_1˜S/L_n are crossed in row and column directions. Thestructure of each unit cell array 20˜23 is shown in FIG. 5. Theplurality of bit lines BL_1˜BL_n are connected one by one to a pluralityof sense amplifiers 30.

FIG. 13 is a diagram illustrating a write operation of the nonvolatileferroelectric memory according to an embodiment of the presentinvention.

In the embodiment, a write operation cycle can be divided into two suboperation regions. That is, the data “1” is written in the first suboperation region, and the data “0” is written in the second suboperation region.

A high voltage is applied to the bit line BL in a predetermined periodwhen the data “1” is required to be preserved. As a result, a value ofthe data “1” written in the first sub operation region can be preservedin the memory cell.

FIG. 14 is a timing diagram illustrating a write operation of high datain the nonvolatile ferroelectric memory device according to anembodiment of the present invention.

First, suppose that the memory cell Q1 shown in FIG. 5 is selected whenthe data “1” is written. A period t0 is defined as a precharge period ofthe memory cell.

In a period t1, when the selecting signals SEL_1 and SEL_2 transit to‘high’, the switching elements N1 and N2 are turned on. As a result, thebit line BL is connected to a source of the memory cell Q1, and thesensing line S/L is connected to a drain of the memory cell Qm.

Here, a plurality of word lines WL_(1˜WL)_m and a plurality of bottomword lines BWL_1˜BWL_m are maintained at a low level. Then, the bit lineBL_1 and the sensing line S/L_1 are maintained at a low state.

In a period t2, the rest of the bottom word lines BWL_2_BWL_(—m) exceptthe bottom word line BWL_1 connected to the selected memory cell Q1transit to ‘high’. As a result, the ground voltage <GND> is applied to adrain and a source of the memory cell Q1.

Next, in a period t3, a negative voltage VNEG is applied to the wordline WL_1 connected to the selected memory cell Q1. In a period t4, thebottom word line BWL_1 transits to ‘high’. As shown in FIG. 3 a, a highvoltage is applied to the ferroelectric layer 16 by voltage division ofthe word line WL_1 and the bottom word line BWL_1, so that the data “1”is written.

In a period t5, the word line WL_1 and the bottom word line BWL_1 aretransited to the ground state, again. In a period t6, the rest of thebottom word lines BWL_2˜BWL_m are transited to the ground state, so thatthe write operation is completed. Thereafter, in a period t7, when theselecting signals SEL_1 and SEL_2 transit to ‘low’, the switchingelements N1 and N2 are turned off.

FIG. 15 is a timing diagram illustrating the write operation of the data“0” and the maintenance operation of the data “1” in the nonvolatileferroelectric memory device according to an embodiment of the presentinvention.

First, suppose that the memory cell Q1 shown in FIG. 5 is selected whenthe data “0” is written. A period t0 is defined as a precharge period ofthe memory cell.

In a period t1, when the selecting signal SEL_1 transits to ‘high’, theswitching element N1 is turned on. As a result, the bit line BL isconnected to the source of the memory cell Q1.

Here, the selecting signal SEL_2, the plurality of word lines WL_1˜WL_mand the plurality of bottom word lines BWL_1˜BWL_m are maintainted atthe low state. The bit line BL_1 and the sensing line S/L_1 aremaintained at the low state.

Thereafter, in a period t2, all bottom word lines BWL_1˜BWL_m transit to‘high’. As a result, all of the memory cells Q1˜Qm are connected to thebit line BL through the bottom word lines BWL_1˜BWL_m, so that dataapplied to the bit line BL can be transmitted to all of the cells Q1˜Qm.

In a period t3, when the data to be written in the memory cell Q1 is“0”, the bit line BL_1 is continuously maintained at the ground voltagestate. On the other hand, the bit line BL_1 transits to ‘high’ when thedata “1” stored in the memory cell Q1 is required to be maintained.

In a period t4, the word line WL_1 transits to ‘high’. As shown in FIG.4 a, electrons are accumulated in the P-type channel region 12 of thememory cell Q1 by the word line WL_1. Then, the positive voltage isapplied to the word line WL_1, and a threshold voltage difference isgenerated. Thus, the polarity is formed so that channel electrons may beinduced to the ferroelectric layer 16. As a result, the data “0” iswritten in the memory cell Q1.

When the data “1” stored in the memory cell Q1 is required to bemaintained, a high voltage is applied to the bit line BL_1, so that avoltage of the bit line BL_1 is applied to the memory cell Q1. As aresult, since the electrons are prevented from being formed in thechannel region 12, the data “1” can be preserved.

Thereafter, in a period t5, the word line WL_1 is transited to theground state. In a period t6, all of the bottom word lines BWL_1˜BWL_mand the bit line BL_1 are transited to the ground state, so the writeoperation is completed. In a period t7, when the selecting signal SEL_1transits to ‘low’, the switching element N1 is turned off.

FIG. 16 is a timing diagram illustrating a sensing operation of celldata in the nonvolatile ferroelectric memory device according to anembodiment of the present invention.

First, suppose that the memory cell Q1 shown in FIG. 5 is selected whenthe data is sensed. A period t0 is defined as a precharge period of thememory cell.

In a period t1, when the selecting signals SEL_1 and SEL_2 transit to‘high’, the switching elements N1 and N2 are turned on. As a result, thebit line BL is connected to the source of the memory cell Q1, and thesensing line S/L is connected to the drain of the memory cell Qm.

Here, the plurality of word lines WL_1˜WL_m and the plurality of bottomword lines BWL_1˜BWL_m are maintained at the low state. The senseamplifier 30, the bit line BL_1 and the sensing line S/L_1 aremaintained at the low state.

Thereafter, in a period t2, the rest of the bottom word linesBWL_2˜BWL_m except the bottom word line BWL_1 connected to the selectedmemory cell Q1 transits to ‘high’ As a result, the rest memory cellsQ2_Qm except the selected memory cell Q1 are connected to the sensingline S/L_1.

Here, the plurality of word lines WL_1˜WL_m are maintained all at theground state. Thus, the flowing of current between the bit line BL_1 andthe sensing line S/L is determined depending on the polarity stateformed in the memory cell Q1.

In a period t3, when the sense amplifier 30 is operated so that asensing voltage is applied to the bit line BL_1, the flowing of currentof the bit line BL_1 is determined depending on the state of the memorycell Q1.

As shown in FIG. 3 b, when current is not applied to the bit line BL_1,it is understood that the data “1” is stored in the memory cell Q1. Onthe other hand, as shown in FIG. 4 b, when a current over apredetermined value is applied to the bit line BL_1, it is understoodthat the data “0” is stored in the memory cell Q1.

In a period t4, when the operation of the sense amplifier 30 is stopped,the bit line BL_1 transits to ‘low’, so that the sensing operation iscompleted. In a period t5, the plurality of bottom word linesBWL_2˜BWL_m transit to ‘low’. In a period t6, when the selecting signalsSEL_1 and SEL_2 transit to ‘low’, the switching elements N1 and N2 areturned off.

Although the floating channel 15 comprising the N-type drain region 13,the P-type channel region 12 and the N-type source region 14 isexemplified here, the present invention is not limited but the floatingchannel layer 15 can comprise a P-type drain region, a P-type channelregion and a P-type source region.

As described above, in an embodiment of the present invention, data of acell are not destroyed at a read mode by using a NDRO (Non DestructiveRead Out). As a result, reliability of the cell can be improved at a lowvoltage of a nano scale ferroelectric cell and a read operation speedcan be also improved. Additionally, a plurality of ferroelectric unitcell arrays are deposited to improve integrated capacity of the cell,thereby reducing the whole size of the cell.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and described in detail herein. However, itshould be understood that the invention is not limited to the particularforms disclosed. Rather, the invention covers all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined in the appended claims.

1. A nonvolatile ferroelectric memory device, comprising: an insulatinglayer formed on a bottom word line; a floating channel layer comprisinga P-type channel region formed on the insulating layer and kept at afloating state, a N-type drain region and a N-type source region whichare connected to both sides of the channel region; a ferroelectric layerformed on the channel region of the floating channel layer; and a wordline formed on the ferroelectric layer, wherein different channelresistance is induced to the channel region depending on polarity statesof the ferroelectric layer, so that data are read and written.
 2. Thenonvolatile ferroelectric memory device according to claim 1, whereinthe floating channel layer is formed of at least one of carbon nanotube, silicon and germanium.
 3. The nonvolatile ferroelectric memorydevice according to claim 1, wherein a channel of the floating channellayer is turned off when positive charges are induced to the channelregion depending on the polarity of the ferroelectric layer to cause ahigh resistance state, and turned on when negative charges are inducedto the channel region depending on the polarity of the ferroelectriclayer to cause a low resistance state.
 4. The nonvolatile ferroelectricmemory device according to claim 1, wherein the floating channel layerwrites high data when positive charges are induced to the channel regionwhile a positive voltage is applied to the bottom word line, a negativevoltage is applied to the word line and a ground voltage is applied tothe drain region and the source region, and the floating channel layerreads high data when the ground voltage is applied to the bottom wordline and the word line so that the channel of the floating channel layeris turned off.
 5. The nonvolatile ferroelectric memory device accordingto claim 1, wherein the floating channel layer writes low data when apositive voltage is applied to the bottom word line and the word lineand a ground voltage is applied to the drain region and the sourceregion so that negative charges are induced to the channel region, andthe floating channel layer reads low data when the ground voltage isapplied to the bottom word line and the word line so that the channel ofthe floating channel layer is turned on.
 6. A nonvolatile ferroelectricmemory device comprising a unit cell array which includes: a pluralityof bottom word lines; a plurality of insulating layers formed on theplurality of bottom word lines, respectively; a floating channel layercomprising a plurality of P-type channel regions located on theplurality of insulating layers and a plurality of N-type drain andsource regions which are alternately connected in series to theplurality of P-type regions; a plurality of ferroelectric layers formedrespectively on the plurality of P-type channel regions of the floatingchannel layer; and a plurality of word lines formed on the plurality offerroelectric layers, respectively, wherein the unit cell array readsand writes a plurality of data by inducing different channel resistanceto the plurality of P-type channel regions depending on polarity statesof the plurality of ferroelectric layers.
 7. The nonvolatileferroelectric memory device according to claim 6, wherein the unit cellarray includes a plurality of layers and each of the plurality of layersis separated by an insulating layer.
 8. A nonvolatile ferroelectricmemory device comprising a unit cell array which includes: a bottom wordline; a plurality of insulating layers formed on the bottom word line,respectively; a floating channel layer comprising a plurality of P-typechannel regions located on the plurality of insulating layers and aplurality of N-type drain and source regions which are alternatelyconnected in series to the plurality of P-type channel regions; aplurality of ferroelectric layers formed respectively on the pluralityof P-type channel regions of the floating channel layer; and a pluralityof word lines formed on the plurality of ferroelectric layers,respectively; wherein the unit cell array reads and writes a pluralityof data by inducing different channel resistance to the plurality ofP-type channel regions depending on polarity states of the plurality offerroelectric layers.
 9. The nonvolatile ferroelectric memory deviceaccording to claim 8, wherein the unit cell array includes a pluralityof layers and each of the plurality of layers is separated by aninsulating layer.
 10. A nonvolatile ferroelectric memory devicecomprising: a bottom word line; a floating channel layer formed on thebottom word line and kept at a floating state; a ferroelectric layer,formed on the floating channel layer, where data are stored; and a wordline formed on the ferroelectric layer in parallel with the bottom wordline, wherein a write operation is performed on data corresponding tothe ferroelectric layer depending on a state of a voltage level appliedto the bottom word line and the word line, and a read operation isperformed on data by inducing different channel resistance to thefloating channel layer depending on polarity states of charges stored inthe ferroelectric layer.
 11. The nonvolatile ferroelectric memory deviceaccording to claim 10, wherein the floating channel layer comprise aP-type channel region, a N-type drain region and a N-type source regionwhich are connected to both sides of the P-type channel region.
 12. Thenonvolatile ferroelectric memory device according to claim 10, whereinthe floating channel layer comprises a P-type channel region, and aP-type drain region and a P-type source region which are connected toboth sides of the P-type channel region.
 13. A nonvolatile ferroelectricmemory device comprising: a plurality of memory cells whose switchingoperations are selectively controlled respectively depending on voltagesapplied to a plurality of word lines and a plurality of bottom wordlines and where floating channel layers are connected serially; a firstswitching element for selectively connecting the plurality of memorycells to a bit line in response to a first selecting signal; a secondswitching element for selectively connecting the plurality of memorycells to a sensing line in response to a second selecting signal,wherein each of the plurality of memory cells comprises: an insulatinglayer formed on the bottom word line; the floating channel layercomprising a P-type channel region formed on the insulating layer andkept at a floating state, a N-type drain region and a N-type sourceregion which are connected to both sides of the channel region; aferroelectric layer formed on the channel region of the floating channellayer; and a word line formed on the ferroelectric layer.
 14. Thenonvolatile ferroelectric memory device according to claim 13, whereinthe first switching element and the second switching element are kept onwhen high data are written in the plurality of memory cells, the firstswitching element is turned on and the second switching element isturned off when low data are written in the plurality of memory cells.15. The nonvolatile ferroelectric memory device according to claim 13,wherein one selected from the plurality of memory cells writes high datawhen a positive voltage is applied to the bottom word line, a negativevoltage is applied to the word line and a ground voltage is applied tothe bit line and the sensing line, and one selected from the pluralityof memory cells writes low data when a positive voltage is applied tothe bottom word line and the word line and a ground voltage is appliedto the bit line and the sensing line.
 16. The nonvolatile ferroelectricmemory device according to claim 15, wherein a plurality of word linesconnected to ones unselected from the plurality of memory cells are keptat a low state and a plurality of bottom word lines are kept at a highstate.
 17. The nonvolatile ferroelectric memory device according toclaim 13, wherein one selected from the plurality of memory cells readshigh data when a ground voltage is applied to the bottom word line andthe word line and the channel of the floating channel layer is turnedoff, and one selected from the plurality of memory cells reads low datawhen a ground voltage is applied to the bottom word line and the wordline and the channel of the floating channel layer is turned on.
 18. Anonvolatile ferroelectric memory device comprising: a plurality of bitlines arranged in a row direction; a plurality of sensing lines arrangedperpendicular to the plurality of bit lines; a plurality of memory cellsarranged in row and column directions where the plurality of bit linesand the plurality of sensing lines are crossed; and a plurality of senseamplifiers connected one by one to the plurality of bit lines, whereineach of the plurality of memory cells comprises: an insulating layerformed on a bottom word line; the floating channel layer comprising aP-type channel region formed on the floating layer and kept at afloating state, a N-type drain region and a N-type source region whichare connected to both sides of the channel region; a ferroelectric layerformed on the channel region of the floating channel layer; and a wordline formed on the ferroelectric layer.
 19. The nonvolatileferroelectric memory device according to claim 18, wherein each of theplurality of memory cells further comprises: a first switching elementfor selectively connecting the plurality of memory cells to a bit linein response to a first selecting signal; and a second switching elementfor selectively connecting the plurality of memory cells to a sensingline in response to a second selecting signal.
 20. The nonvolatileferroelectric memory device according to claim 19, wherein the firstswitching element and the second switching element are kept on when highdata are written in the plurality of memory cells, the first switchingelement is turned on and the second switching element is turned off whenlow data are written in the plurality of memory cells.
 21. Thenonvolatile ferroelectric memory device according to claim 20, whereinone selected from the plurality of memory cells writes high data when apositive voltage is applied to the bottom word line, a negative voltageis applied to the word line and a ground voltage is applied to the bitline and the sensing line, and one selected from the plurality of memorycells writes low data when a positive voltage is applied to the bottomword line and the word line and a ground voltage is applied to the bitline and the sensing line.
 22. The nonvolatile ferroelectric memorydevice according to claim 19, wherein a ground voltage is applied to thebottom word line and the word line when the first switching element andthe second switching element are turned on at the read mode of theplurality of memory cells.